The regulation of de novo synthesis of thyroid hormones in primary cultures of human thyroid cells has been examined and correlated with the regulation of the synthesis of the insulin-like growth factor-binding proteins (IGFBPs). In the serum-free culture medium, insulin and TSH (0.01-0.3U/L)were found to be obligatory additives for iodide uptake and organification. In the presence of TSH, cells reorganized into 3D follicles, which stored thyroglobulin. High concentrations of TSH ( > 1U/L), epidermal growth factor, protein kinase C activation with phorbol esters, and transforming growth factor beta 1 all were strongly inhibitory to iodide metabolism and thyroid hormone synthesis. Conditioned medium from the thyroid cell cultures contained at least 5 125I-IGF-labeled bands IGFBPs, including the two glycosylation variants of IGFBP-3. TSH, at concentrations optimal for iodide uptake, inhibited the secretion of all these binding proteins. These effects were mimicked by forskolin and the cell-permeable analog of cAMP, dibutyryl cAMP. The changes in IGFBP proteins were reflected by marked reductions in the steady-state levels of the messenger RNAs of IGFBP-3 and IGFBP-5. This reduction was less pronounced for IGFBP-4. In contrast, protein kinase C activation with phorbol esters and transforming growth factor beta, and high TSH concentrations enhanced IGFBP secretion. Steady-state levels of IGFBP-3 and IGFBP-5 messenger RNAs were elevated after treatment with transforming growth factor-beta and high TSH concentrations. This Study shows that enhanced production of IGFBPs is correlated with inhibition of thyroid function and that TSH, through cAMP, is one factor capable of inhibiting IGFBP production.